The present invention generally relates to record track identification and following systems of the type employing a magnetic record medium having a plurality of record tracks and a transducer moved transversely with respect to the record tracks, first in a search mode to identify a desired record track, and then in a servo mode to maintain the transducer centered with respect to the selected record track. More especially, the present invention has a particular application to flexible or floppy disc memories in which the record medium is in the form of a circular disc having a plurality of concentric data tracks on at least one surface of the disc. In such a disc memory, a transducer is moved radially by a positioning apparatus in order first to locate a desired record track and thereafter to follow the selected record track.
One of the problems encountered in the use of flexible discs is the difficulty of keeping the transducer on the record track which has been selected. The reason this is a problem is that, even though the record tracks are recorded in concentric circles on the circular disc, the circular tracks, because of the flexibility of the disc and variations in temperature, atmospheric pressure, mechanical forces and the like, become actually oval or elliptical and do not remain circular. Therefore, the transducer cannot simply be positioned on a desired record track and assumed to follow that track as the disc rotates as with rigid disc memories. Thus, it becomes necessary to provide a servoing system which is effective to maintain the transducer centered with respect to a record track.
Typically, the flexible disc is provided with a plurality of servo sector areas which divide the record tracks into a plurality of data fields. The servo sectors contain information for synchronization, track following and track identification, the latter being in the form of addresses. Generally, the more information required in the servo sector areas, the less area remains for the data fields. Moreover, track following information in the servo sectors is often recorded in offset tracks, sometimes requiring increased separation of data tracks in the radial direction of the disc.